Regioselective Annulation of Aryl Sulfonamides with Allenes through Cobalt-Promoted C-H Functionalization

Article abstract of DOI:10.1021/acs.orglett.6b03510

The development of an efficient method for the construction of biologically relevant sultams is described, which represents the first case of cobalt-promoted C-H/N-H functionalization of sulfonamides with allenes. This newly developed annulation reaction demonstrated good functional group tolerance and excellent regioselectivity. Both terminal monosubstituted allenes and internal disubstituted allenes can be employed to give the desired sultams in good yields. This strategy can be successfully used to build a unique sultam library with novel structural diversity.

Full text of DOI:10.1021/acs.orglett.6b03510

Letter
pubs.acs.org/OrgLett
Regioselective Annulation of Aryl Sulfonamides with Allenes through
Cobalt-Promoted C−H Functionalization
Tianlong Lan,† Liguo Wang,† and Yu Rao*
MOE Key Laboratory of Protein Sciences, School of Pharmaceutical Sciences, Tsinghua University, Beijing 100084, China
S
* Supporting Information
ABSTRACT: The development of an efficient method for the
construction of biologically relevant sultams is described, which
represents the first case of cobalt-promoted C−H/N−H functionalization
of sulfonamides with allenes. This newly developed annulation reaction
demonstrated good functional group tolerance and excellent regiose-
lectivity. Both terminal monosubstituted allenes and internal disub-
stituted allenes can be employed to give the desired sultams in good
yields. This strategy can be successfully used to build a unique sultam
library with novel structural diversity.
yclic sulfonamides (sultams) are an important class of
reactions generating sultams through transition-metal-catalyzed
ortho C−H activation of aryl sulfonamides with alkynes and
alkenes have been developed.9 For example, in 2012, the
Cramer9a group reported the synthesis of sultam scaffolds
through Rh-catalyzed C−H functionalization of acylated aryl
sulfonamides with alkynes. In 2014, Li and co-workers9b
described the synthesis of five-membered sultams via Rh-
catalyzed annulation reaction between acylated aryl sulfonamides
and alkenes. More recently, the groups of Ribas9c and
Sundararaju9d independentlyintroducedtheannulationreactions
of aryl sulfonamides with alkynes through cobalt10,11catalysis. It
should be noted that these methods generally employ alkynes and
alkenes as coupling partners. In contrast, the practice of using
allenes as annulation partners in C−H functionalization is still
rare.12 Different from alkenes and alkynes, the complexity of two
orthogonal carbon−carbon double bonds of allenes can provide
chances to access structurally unique products.
C
chemicals that serve as structural motifs in different drugs
and bioactive compounds (Figure 1).1 For example, the attractive
Figure 1. Drugs containing sultam motifs.
Because of the special reactivity of the 1,2-diene functionality,
allenes have demonstrated great potential in organic chemistry.13
In particular, the carbometalation reaction of allenes recently is
one of the most interesting research field and has exhibited high
potential for the preparation of novel and diverse scaffolds.14
However, to the best of our knowledge, there are no reported
examples of cobalt-promoted C(sp2)−H functionalization of
sulfonamides with allenes. We envisioned that the cobalt-
promoted ortho C(sp2)−H activation of sulfonamides might
undergo an annulation reaction with allenes to produce
corresponding sultams. Herein, we report the first example of
regioselective sultam synthesis through cobalt-promoted C−H/
N−H annulation of aryl sulfonamides with allenes (Scheme 1).
Initially, we began our investigations by screening different
cobalt reagents in the reaction system, which consists of
sulfonamide 1a, allene partner 2a, Mn(OAc)2, KOAc, and
biological activities of sultams have been found in a wide range of
fields including anti-inflammatory,2 antibacterial,3 anti-HIV,4 and
antimalarial,5 etc.6 Because of its significance in pharmaceutical
development, a number of strategies have been developed for the
synthesis of sultams.7 However, most of the reported methods
rely on intramolecular cyclization of elaborate precursors or
intermolecular reactions starting from prefunctionalized starting
materials. These methods usually suffer from multistep synthesis
of precursors, tedious procedures, and low yields. Thus, further
development of new, efficient, and general methods toward the
synthesis of cyclic sulfonamides using readily available precursors
is quite appealing.
In recent years, transition-metal-catalyzed annulation reactions
throughC−Hbondfunctionalization havegraduallyemergedasa
powerful tool for the synthesis of various heterocyclic
compounds.8 The straightforward synthesis of sultam scaffolds
from simple starting materials through C−H functionalization
reactions would be very attractive. To date, several annulation
Received: November 23, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Ortho C−H Functionalization of Aryl Sulfonamides
Scheme 2. Reaction Scope of Aryl Sulfonamides
trifluoroethanol (TFE) under air at 100 °C for 16 h. We found
that Co(OAc)2·4H2O was the optimal promoter among the four
cobalt sources tested (Table 1, entries 1−5). When Co(OAc)2·
4H2O was used, the desired sultam product 3a was observed in
13% NMR yield (entry 5; for the X-ray crystal structure, see the
Supporting Information). Changing the oxidant from Mn(OAc)2
to Mn(OAc)3·2H2O resulted in an improved 35% NMR yield
(entry 6). To our delight, we found that O2 could promote the
reaction significantly (entries 5, 7, and 12). Furthermore, a higher
isolated yield (70%) can be obtained by replacing KOAc with
NaOPiv·H2O (entry 10). For all of the conditions examined in
Table 1, no obvious regioisomers can be observed. The control
experiment showed that the omission of cobalt source resulted in
the complete inactivity of the reaction (entry 14).
a
Reaction conditions: aryl sulfonamide (0.1 mmol), 2a (3.0 equiv),
Co(OAc)2·4H2O (20−50 mol %; in some cases Co(NO3)2·6H2O was
used instead of Co(OAc)2·4H2O), Mn(OAc)3·2H2O (2.0 equiv),
NaOPiv·H2O (2.0 equiv), 1.5 mL of TFE, 100 °C, O2.
With the optimal conditions in hand, we next attempted to
explore the substrate scope for this new reaction. A variety of
substituted sulfonamides bearing 8-aminoquinoline directing
groups were tested under the optimal conditions (Scheme 2).
Electron-rich substrates (3a−e) could proceed smoothly to
furnish the desired products in good isolated yields, whereas
electron-deficient substrates (3h−k) only gave low to moderate
isolated yields under the standard conditions with almost half of
the starting material remaining. That may attribute to the
increaseddifficultyoftheC−Hactivationstepcausedbyelectron-
withdrawing groups. To our satisfaction, these electron-deficient
substrates could provide products in good yields when the
Co(OAc)2·4H2O loading was increased from 20 to 50 mol %.
Functional groups such as −NHAc, −Cl, and −Br were well
tolerated. Notably, some heterocyclic substrates were found to be
compatible with the reaction system as well. For example, the 2-
thiophenesulfonamide substratecanbereadilytransformedtothe
a
Table 1. Optimization of Cobalt-Promoted C−H Activation and Annulation with Allene
b
entry
cobalt source
oxidant
Mn(OAc)2
base
KOAc
yield of 3a (%)
1
2
Co(acac)2
<5
<5
<5
<5
13
35
40
Co(acac)3
Mn(OAc)2
KOAc
3
CoF3
Mn(OAc)2
KOAc
4
CO(NO3)2·6H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(OAc)2·4H2O
Co(NO3)2·6H2O
Mn(OAc)2
KOAc
5
Mn(OAc)2
KOAc
6
Mn(OAc)3·2H2O
Mn(OAc)2/O2
KOAc
7
KOAc
e
8
Mn(OAc)3·2H2O/O2
Mn(OAc)2/O2
KOAc
59
e
9
NaOPiv·H2O
NaOPiv·H2O
NaOPiv·H2O
NaOPiv·H2O
NaOPiv·H2O
NaOPiv·H2O
68
e
10
Mn(OAc)3·2H2O/O2
Mn(OAc)3·2H2O/O2
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O/O2
Mn(OAc)3·2H2O/O2
70
c
e
11
d
64
12
51
e
13
14
69
0
a
Reaction conditions: 1a (0.1 mmol), 2a (3.0 equiv), cobalt (20 mol %), oxidant (2.0 equiv), base (2.0 equiv), 1.5 mL of TFE (trifluoroethanol),
b
c
1
100 °C, 16 h, air. Yields are calculated by H NMR analysis of the reaction mixture using p-nitrobenzaldehyde as internal standard. 85 °C, 16 h.
d
e
N2. Isolated yields.
B
DOI: 10.1021/acs.orglett.6b03510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
desired sultam (3l), which could be regarded as an analogue of
brinzolamide. It is worth noting that in our case the 2-
pyrrolesulfonamide substrate is well tolerated, even though the
substrate contains the free N−H group.
alkynes, which may provide two regioisomers with terminal
aliphatic alkynes,9a,d only one regioisomer can be observed from
this reaction in all cases. It should be mentioned that all of the
allenes used can be easily accessed on large scale through
inexpensive and simple starting materials in one or two steps,
except 2e, which takes three steps for preparation. To further
evaluate the practical utility of this new reaction, a larger scale
reaction was performed by using 1a (2 mmol, 600 mg) and allene
partner 2h, which gave 4ah in 70.5% yield (see Scheme 4).
Utilizing this annulation reaction, we then turned our attention
to explore the scope and reactivity pattern of the allene partners.
As shown in Scheme 3, both electron-poor and electron-rich
Scheme 3. Reaction Scope of Allenes*
Scheme 4. Sub-Gram-Scale Synthesis of 4ah
As illustrated in Scheme 5, a plausible reaction mechanism was
proposed. The first step involves the oxidation of Co(II) to give
Scheme 5. Proposed Reaction Pathway
*
Reaction conditions: aryl sulfonamide (0.1 mmol), allene (2b−g)
(3.0 equiv), Co(OAc)2·4H2O (20−50 mol %, in some cases
Co(NO3)2·6H2O was used instead of Co(OAc)2·4H2O), Mn(OAc)3·
2H2O (2.0 equiv), NaOPivH2O (2.0 equiv), 1.5 mL of TFE, 100 °C,
a
O2. Product derived from 2e.
Co(III) by Mn(OAc)3. Next, chelation of Co(III) to the
quinoline directing group and subsequent C−H activation gives
intermediate A. The following step is the coordination of the
allene to the Co(III) center of intermediate A to yield the
intermediate B. Based on the relevant reports of allene-involved
C−H functionalization12g,15 and cobalt-catalyzed alkyne annula-
tion,9c,d,16 in which vinylmetal intermediates were proposed, the
next step might be the insertion of the less hindered double bond
of the allene into the carbon−cobalt bond to form intermediate C
in this case. Following this step, the reductive elimination of
intermediate C gives Co(I) and intermediate D. Intermediate D
could be isolated and identified (for details, see Supporting
Information). Finally, intermediate D undergoes 1,3-H shift to
provide the desired product 3a.17 The Co(I) can be further
oxidized to Co(III) for the next turnover. O2 is likely to play a role
as the terminal oxidant in the entire process.
In conclusion, we have established a new, effective method for
the construction of biologically relevant sultam scaffolds that
represents the first example of cobalt-promoted C−H/N−H
functionalization of aryl sulfonamides with allenes. A broad range
of aryl sulfonamides can be exploited to react with both terminal
monosubstituted allenes and internal disubstituted allenes to give
the desired sultams in good yields with excellent regioselectivity.
allenes (2b−g) were examined in the reaction system. It was
found that the electronic properties of the allenes do not change
the fashion of the annulation reaction. The new C−C bond
formed preferentially between the ortho carbon of the aryl
sulfonamides and the less hindered terminal carbon of the allenes.
Undertheseconditions, alltestedallenescouldberegioselectively
incorporated to generate the desired sultams in good isolated
yields. For the internal disubstituted allene 2b, the small steric
hindrance difference between the methyl group and the ester
group can be accurately recognized. It was interesting that when
2e was used as the allene partner, 1,3-diene derivatives 4ae−je
formed as the final products. When 2g was used as the allene
partner, partial elimination led to a mixture of product (4ag) and
the 1,3-diene derivative (4ag′) in a ratio of 3:1. This sequential
reaction enriches the diversity of products derived from the
annulationreaction. Satisfyingly, thenine-membered cyclicallene
2f is also compatible with the reaction system to furnish the three-
ring-fused heterocycle products 4af−lf, which are difficult to
obtain through known methods of sultam synthesis. This new
reaction demonstrates excellent regioselectivity. Compared with
rhodium-orcobalt-catalyzedannulationofarylsulfonamideswith
C
DOI: 10.1021/acs.orglett.6b03510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(k) Rayabarapu, D. K.; Zhou, A.; Jeon, K. O.; Samarakoon, T.; Rolfe, A.;
Siddiqui, H.; Hanson, P. R. Tetrahedron 2009, 65, 3180−3188. (l) Miura,
T.; Yamauchi, M.; Kosaka, A.; Murakami, M. Angew. Chem., Int. Ed. 2010,
49, 4955−4957. (m) Kaneko, K.; Yoshino, T.; Matsunaga, S.; Kanai, M.
Org. Lett. 2013, 15, 2502−2505.
(8) For reviews andexamples, see: (a) Liu, Z.; Zhang, X.; Larock, R. C. J.
Am. Chem. Soc. 2005, 127, 15716−15717. (b) Liu, Z.; Wasmuth, A. S.;
Nelson, S. G. J. J. Am. Chem. Soc. 2006, 128, 10352−10353. (c) Gorin, D.
J.; Watson, I. D. G.; Toste, F. D. J. Am. Chem. Soc. 2008, 130, 3736−3737.
(d) Jin, T.; Zhao, J.; Asao, N.; Yamamoto, Y. Chem. - Eur. J. 2014, 20,
Upon successful implementation, this strategy can be further
employedtobuildapromisingsultamlibrarywithnovelstructural
diversity for biological activity screening.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b03510.
General experimental procedure, characterization data for
the products, and X-ray crystal data for 3a (PDF)
3554−3576. (e)Gulías, M.;Mascarenas, J. L. Angew. Chem., Int. Ed. 2016,
̃
55, 11000−11019.
(9) (a) Pham, M. V.; Ye, B.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51,
10610−10614. (b) Xie, W.; Yang, J.; Wang, B.; Li, B. J. Org. Chem. 2014,
79, 8278−8287. (c) Planas, O.; Whiteoak, C. J.; Company, A.; Ribas, X.
Adv. Synth. Catal. 2015, 357, 4003−4012. (d) Kalsi, D.; Sundararaju, B.
Org. Lett. 2015, 17, 6118−6121.
(10) For selected reviews of cobalt-catalyzed C−H functionalization,
see: (a) Yoshikai, N. Synlett 2011, 2011, 1047−1051. (b) Ackermann, L.
J. Org. Chem. 2014, 79, 8948−8954. (c) Gao, K.; Yoshikai, N. Acc. Chem.
Res. 2014, 47, 1208−1219. (d) Gandeepan, P.; Cheng, C. Acc. Chem. Res.
2015, 48, 1194−1206. (e)Moselage, M.;Li, J.; Ackermann, L. ACSCatal.
2016, 6, 498−525.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yrao@mail.tsinghua.edu.cn.
ORCID
Tianlong Lan: 0000-0002-9435-9959
Author Contributions
†T.L. and L.W. contributed equally.
Notes
(11) For selected examples of cobalt-catalyzed annulation reactions,
see: (a) Ikemoto, H.; Yoshino, T.; Sakata, K.; Matsunaga, S.; Kanai, M. J.
Am. Chem. Soc. 2014, 136, 5424−5431. (b) Grigorjeva, L.; Daugulis, O.
Org. Lett. 2014, 16, 4684−4687. (c) Grigorjeva, L.; Daugulis, O. Angew.
Chem., Int. Ed. 2014, 53, 10209−10212. (d) Ma, W.; Ackermann, L. ACS
Catal. 2015, 5, 2822−2825. (e) Li, J.; Tang, M.; Zang, L.; Zhang, X.;
Zhang, Z.;Ackermann, L. Org. Lett. 2016, 18, 2742−2745. (f)Nguyen, T.
T.; Grigorjeva, L.; Daugulis, O. ACS Catal. 2016, 6, 551−554. (g) Hao,
X.; Du, C.; Zhu, X.; Li, P.; Zhang, J.; Niu, J.; Song, M. Org. Lett. 2016, 18,
3610−3613. (h) Mei, R.; Wang, H.; Warratz, S.; Macgregor, S. A.;
Ackermann, L. Chem. - Eur. J. 2016, 22, 6759−6763. (i) Sivakumar, G.;
Vijeta, A.; Jeganmohan, M. Chem. - Eur. J. 2016, 22, 5899−5903.
(j) Kong, L.; Yu, S.; Zhou, X.; Li, X. Org. Lett. 2016, 18, 588−591.
(k) Liang, Y.; Jiao, N. Angew. Chem., Int. Ed. 2016, 55, 4035−4039.
(12) (a) Kuninobu, Y.; Yu, P.; Takai, K. Org. Lett. 2010, 12, 4274−4276.
(b) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 8181−8184.
(c) Ochi, Y.; Kurahashi, T.; Matsubara, S. Org. Lett. 2011, 13, 1374−
1377. (d) Wang, H.; Glorius, F. Angew. Chem., Int. Ed. 2012, 51, 7318−
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 21302106, 81573277, and
81622042) and Tsinghua University Initiative Scientific Research
Program. We thank Lingpeng Zhan, Prof. Zongxiu Nie (Institute
of Chemistry, Chinese Academy of Sciences, Beijing), and Zhihui
Sang for their help.
REFERENCES
■
(1) (a) Rabasseda, X.; Hopkins, S. J. Drugs Today 1994, 30, 557−564.
(b) Wroblewski, T.; Graul, A.; Castaner, J. Drugs Future 1998, 23, 365−
369. (c) Wells, G. J.; Tao, M.; Josef, K. A.; Bihovsky, R. J. Med. Chem.
2001, 44, 3488−3503.
(2) (a) Lombardino, J. G.; Wiseman, E. H.; Mclamore, W. M. J. Med.
Chem. 1971, 14, 1171−1175. (b) Lombardino, J. G.; Wiseman, E. H. J.
Med. Chem. 1972, 15, 848−849. (c) Nicolas, C.; Verny, M.; Giraud, I.;
Ollier,M.;Rapp,M.;Maurizis,J.C.;Madelmont, J.C. J.Med.Chem. 1999,
42, 5235−5240. (d) Xu, S.; Rouzer, C. A.; Marnett, L. J. IUBMB Life
2014, 66, 803−811.
7322. (e) Rodríguez, A.; Albert, J.; Ariza, X.; Garcia, J.; Granell, J.; Farras
̀
,
J.; La Mela, A.; Nicolas. J. Org. Chem. 2014, 79, 9578−9585. (f) Xia, X.;
́
Wang, Y.; Zhang, L.; Song, X.; Liu, X.; Liang, Y. Chem. - Eur. J. 2014, 20,
5087−5091. (g) Thrimurtulu, N.; Dey, A.; Maiti, D.; Volla, C. M. R.
Angew. Chem., Int. Ed. 2016, 55, 12361−12365. (h) Li, T.; Zhang, C.;
Tan, Y.; Pan, W.; Rao, Y. Org. Chem. Front. 2017, 4, 204−209.
(3) Zia-ur-Rehman, M.; Choudary, J. A.; Ahmad, S.; Siddiqui, H. L.
Chem. Pharm. Bull. 2006, 54, 1175−1178.
(13) For selected reviews, see: (a) Ma, S. Aldrichimica Acta 2007, 40,
91−102. (b) Yu, S.; Ma, S. Angew. Chem., Int. Ed. 2012, 51, 3074−3112.
(c) Ye, J.; Ma, S. Acc. Chem. Res. 2014, 47, 989−1000. (d) Le Bras, J.;
Muzart, J. Chem. Soc. Rev. 2014, 43, 3003−3040. (e) Alcaide, B.;
Almendros, P.; Aragoncillo, C. Chem. Soc. Rev. 2014, 43, 3106−3135.
(f) Neff, R. K.; Frantz, D. E. Tetrahedron 2015, 71, 7−18.
(4) Zhuang, L.; Wai, J. S.; Embrey, M. W.; Fisher, T. E.; Egbertson, M.
S.;Payne, L. S., Jr; Guare, J.; Vacca, J. P.; Hazuda, D. J.; Felock, P. J. J. Med.
Chem. 2003, 46, 453−456.
(5) Valente, C.; Guedes, R. C.; Moreira, R.; Iley, J.; Gut, J.; Rosenthal, P.
J. Bioorg. Med. Chem. Lett. 2006, 16, 4115−4119.
(14) For selected examples of carbometalation reactions, see:
(a) Larock, R. C.; Berrios-Pena, N. G.; Fried, C. A. J. Org. Chem. 1991,
56, 2615−2617. (b) Larock, R. C.; Zenner, J. M. J. Org. Chem. 1995, 60,
(6) (a) Scozzafava, A.; Owa, T.; Mastrolorenzo, A.; Supuran, C. T. Curr.
Med. Chem. 2003, 10, 925−953. (b) Chen, X.; Zhang, S.; Yang, Y.;
Hussain, S.; He, M.; Gui, D.; Ma, B.; Jing, C.; Qiao, Z.; Zhu, C. Bioorg.
Med. Chem. 2011, 19, 7262−7269.
482−483. (c) Casanova, N.; Seoane, A.; Mascarenas, J. L.; Gulías, M.
̃
Angew. Chem., Int. Ed. 2015, 54, 2374−2377.
(7) (a) Ahn, K. H.; Baek, H.; Lee, S. J.; Cho, C. J. Org. Chem. 2000, 65,
7690−7696. (b) Greig, I. R.; Tozer, M. J.; Wright, P. T. Org. Lett. 2001, 3,
369−371. (c) Chiacchio, U.; Corsaro, A.; Rescifina, A.; Bkaithan, M.;
Grassi, G.; Piperno, A.; Privitera, T.; Romeo, G. Tetrahedron 2001, 57,
3425−3433. (d) Lee, J.; Zhong, Y.; Reamer, R. A.; Askin, D. Org. Lett.
2003, 5, 4175−4177. (e) Layman, W. J.; Greenwood, T. D.; Downey, A.
L.; Wolfe, J. F. J. Org. Chem. 2005, 70, 9147−9155. (f) Enders, D.; Moll,
A.; Bats, J. W. Eur. J. Org. Chem. 2006, 2006, 1271−1284. (g) Liu, X.; Li,
C.; Che, C. Org. Lett. 2006, 8, 2707−2710. (h) Vasudevan, A.; Tseng, P.;
Djuric, S. W. Tetrahedron Lett. 2006, 47, 8591−8593. (i) Sherman, E. S.;
Fuller, P. H.; Kasi, D.; Chemler, S. R. J. Org. Chem. 2007, 72, 3896−3905.
(15)Forselectedexamples, see:(a)Zeng, R.;Fu, C.;Ma, S. J. Am. Chem.
Soc. 2012, 134, 9597−9600. (b) Zeng, R.; Wu, S.; Fu, C.; Ma, S. J. Am.
Chem. Soc. 2013, 135, 18284−18287. (c) Nakanowatari, S.; Ackermann,
L. Chem. - Eur. J. 2015, 21, 16246−16251. (d) Wu, S.; Zeng, R.; Fu, C.;
Yu, Y.; Zhang, X.; Ma, S. Chem. Sci. 2015, 6, 2275−2285.
(16) Grigorjeva, L.; Daugulis, O. Angew. Chem., Int. Ed. 2014, 53,
10209−10212.
(17) The transformation could be observed even in CDCl3 at room
temperature (for details, see the Supporting Information).
́
(j) Jimenez-Hopkins, M.; Hanson, P. R. Org. Lett. 2008, 10, 2223−2226.
D
DOI: 10.1021/acs.orglett.6b03510
Org. Lett. XXXX, XXX, XXX−XXX